Design parameters for urban drainage systems in Belgium have been revised. The revision involves extrapolation of the design rainfall statistics, taking into account the current knowledge on future climate change trends till 2100. Uncertainties in these trend projections have been assessed after statistically analysing and downscaling a broad ensemble set of 44 regional and 69 global climate model runs. Climate change scenarios were developed, tailored for the application of urban drainage impact analysis. By means of a further advanced quantile perturbation method, a 100-year 10-minute historical rainfall series was perturbed for each climate change scenario. The perturbations were based on the empirical frequency distributions for the lower return periods, and calibrated extreme value distributions for the higher return periods.
Based on the climate scenarios and perturbed historical rainfall series, changes in intensity-duration-frequency (IDF) statistics and design storms have been derived. It was shown that the 10-year design storm intensity can increase up to about 50% by the end of this century. Or, systems currently designed for a 20-year return period of flooding, might flood with a mean recurrence interval of – in order of magnitude – 5 years by the end of the century. Based on a continuous reservoir simulation approach, changes in overflow frequency of storage facilities have been quantified for a wide range of storage capacities and infiltration / throughflow rates. It is found that increase in storage capacity of 11% to 51% is required to keep the overflow frequency to the current level.